In response, the Klingons massed a huge amount of ships, troops, and resources, committing approximately one third of all standing Klingon forces, and launched a preemptive strike against the Cardassian Union. Unable to stand against the Klingons in pitched battle, the Cardassians tied down Klingon forces in hit-and-run attacks.

The tide of the war turned in 2373, when GulDukat seized power in a coup d'état, and the Cardassian Union formally joined the Dominion. In response, a fleet of fifty Dominion warships began fortifying Cardassian space. Within days, the Klingons suffered severe casualties and were forced to affect a complete withdrawal, while the Maquis were all but annihilated as a viable military threat.

The manipulation of the Klingon Empire into launching an unprovoked attack, the subsequent destabilization of the Alpha Quadrant political landscape, and the entry of the Cardassian Union into the Dominion proved to be one of the greatest coups of Changeling infiltration in the Alpha Quadrant, and paved the way for the Dominion War several months later.

On Qo'noS, the news was received very differently. ChancellorGowron and an overwhelming majority in the Klingon High Council began to grow concerned that the revolt on Cardassia had been instigated by the Dominion, and the Detapa Council had been replaced by Changeling infiltrators. One of the most vocal proponents of this theory was General Martok, who, unbeknownst to the Klingons, had been replaced by a Changeling. Martok called for a massive invasion of Cardassian space to remove the "Changeling threat". Many Klingons, who had grown restless after decades of peace, rallied around him.

One-third of the Empire's standing forces were to be committed to the invasion. The plan was to mount a preemptive strike into the heart of Cardassian territory, intending to catch the Cardassians off-guard, and conquer Cardassia itself. All government officials would then be executed, and an imperial overseer installed to quell further resistance. An advance force of fifty warships, led by Martok aboard the Klingon flagship, IKS Negh'Var, was dispatched to FederationstarbaseDeep Space 9 under the guise of defending the station against a perceived Dominion incursion.

CaptainBenjamin Sisko, commanding officer of Deep Space 9, was suspicious of the Klingons' motives. There had been no significant Dominion activity near the Bajoran wormhole for several months. Thus, he ordered Lieutenant CommanderWorf to uncover the Klingons' true intentions. Once appraised of the planned invasion, Sisko urged Martok to call off the attack, informing him the Federation could not support an unprovoked invasion. Martok ignored the warnings, and shortly thereafter, the invasion commenced.

The Federation Council publicly condemned the invasion, and in response, Chancellor Gowron withdrew the Empire from the Khitomer Accords, recalled his ambassadors, and expelled all Federation citizens from the Klingon Empire, thereby ending the formal alliance between the two superpowers. Though an overwhelming majority of Klingons supported the war, not all did, however, with EmperorKahless himself publicly denouncing the invasion. (DS9: "The Sword of Kahless")

Wreckage of the Cardassian fleet

Starfleet was under orders not to interfere until the Federation Council could re-establish a political dialogue with Gowron. In the meantime, the Klingons were able to cross into Cardassian space unopposed. However, Elim Garak, aboard Deep Space 9, was able to send a warning to Gul Dukat, chief military advisor to the Detapa Council. Although the Cardassians were able to mobilize, the warning did not give them enough time to mount an effective resistance.

The outlying colonies were rapidly overwhelmed, and the bulk of the Cardassian fleet was destroyed in pitched battle, leaving the Klingon fleet only fifty-two hours from Cardassia. Realizing the Cardassians had little time to mount an effective defense, Sisko offered to shelter the members of the Detapa Council aboard Deep Space 9. Dukat evacuated the Cardassian government aboard the cruiser Prakesh. The Defiant was able to rescue the Prakesh after it was intercepted and severely damaged by Klingon ships.

With this development, Martok turned the Klingon advance force away from Cardassia, and launched an attack against Deep Space 9, with the intent on recovering the Detapa Council members and executing them. In preparation for a Dominion attack, the station's defenses had been heavily augmented, and it was able to withstand the assault long enough for Starfleet reinforcements to arrive. Faced with the prospect of a war on two fronts, Chancellor Gowron ordered the fleet to stand down, against the vehement objections of Martok.

Following their defeat at Deep Space 9, the momentum of the Klingon invasion had been blunted. Cardassia was now out of danger of conquest. Gowron, fearing assassination attempts from his failure to rapidly subdue the planet, simply declared victory and recalled his forces. However, the Klingon forces refused to relinquish the colonies they had conquered, and began to fortify their positions, preparing for a war of attrition as well as launching several attacks on Romulan targets on the Klingon-Romulan border. (DS9: "The Way of the Warrior", "Hippocratic Oath")

In the months following the invasion, neither side ever mustered the strength to hazard out pitched battles and deliver a decisive blow. As a result, the war rapidly degenerated into attrition. Cloaked Klingon warships, operating alone or in small groups, rampaged throughout Cardassian space, destroying targets of opportunity and plundering Cardassian trade routes. Outgunned by the Klingons, the Cardassians responded with unconventional tactics, utilizing sensor ghosts and holographic projections to fool Klingon ships, earning the respect of many Klingon troops for their ingenuity. (DS9: "Return to Grace", "Soldiers of the Empire")

The privateer nature of the Klingon attacks devastated the Cardassian industrial infrastructure, resulting in a massive humanitarian crisis. The Cardassian healthcare system had been all but destroyed; diseases once under control, such as Rudellian plague, swept through Cardassian colonies, while the government struggled to meet even the most basic needs of the people. With their fleet occupied, the Detapa Council was forced to ask Starfleet for assistance, such as in providing escort for medical relief convoys.

With the Cardassians unable to commit resources to combat them, and now equipped with Class 4 industrial replicators intended to help rebuild a whole empire, the Maquis were virtually granted free reign in the Demilitarized Zone. The Maquis entered into an informal alliance with the Klingons, who supplied them with technology such as cloaking devices and weapons. What little remained of the Cardassian military in the Zone were rapidly forced out by the Maquis. (DS9: "Return to Grace", "Rules of Engagement", "For the Cause")

The loss of the Demilitarized Zone to the Maquis, and the severe damage suffered at the hands of Klingon raiders had eroded the willingness of the Detapa Council to persist in the hopes of a military victory. Even after Gul Dukat, with the assistance of MajorKira Nerys, captured a Klingon Bird-of-Prey with a trove of military intelligence, he was unable to convince the Council to go on the offensive, who hoped for a diplomatic solution to the war. (DS9: "Return to Grace")

As a result of the severe political fallout from the Federation's condemnation of the Klingon invasion, the Klingon attack against Deep Space 9, Klingon support for the Maquis and further political manipulation by General Martok, tensions rose dramatically between the Federation and the Klingon Empire. After the Federation refused to cede to an ultimatum to withdraw from the Archanis sector, to which the Klingons had laid claim, Klingon forces invaded Federation territory, widening the war onto two fronts, officially entering the Federation as a Cardassian co-belligerent.

Committed on two fronts, both wars degenerated into a series of savage skirmishes and attrition combat. With no end in sight, and the Detapa Council unwilling to risk a single decisive engagement, Gul Dukat began secret negotiations with the Dominion. In mid-2373, a large Jem'Hadar fleet of some fifty ships crossed through the wormhole and entered Cardassian space, formalizing Cardassia's entry into the Dominion, dramatically shifting the Alpha Quadrant political landscape. As per their treaty, Gul Dukat was placed at the head of the new Cardassian government.

In just three days, the Jem'Hadar inflicted heavy casualties on the Klingon forces occupying Cardassian territory, sweeping Cardassian space clear of all invading ships. In response, Chancellor Gowron issued orders for a complete military withdrawal from Cardassian territory, intending to fortify the Empire and fight the Dominion to the death.(DS9: "In Purgatory's Shadow", "By Inferno's Light")

The Klingons weren't the Jem'Hadars' only targets; after forcing Gowron's retreat, they turned their attention to the Demilitarized Zone and annihilated all Maquis settlements, thereby ending their existence as a viable military threat. (DS9: "Blaze of Glory")

Faced with the overwhelming power of the Dominion military, Chancellor Gowron was persuaded by Captain Sisko to re-instate the Khitomer Accords between the Federation and the Klingon Empire, formally ending the concurrent Federation-Klingon War. After over a year of conflict, the territories of the Klingon Empire, the Cardassian Union, and the United Federation of Planets had returned to their pre-war state. The Klingon Defense Force had been substantially weakened after fighting multiple foes, and agreed to a permanent presence on Deep Space 9 to guard the wormhole. (DS9: "By Inferno's Light")

As a result of the severe damage the Cardassian Union suffered at the hands of the Klingons, the Dominion scheduled weekly convoys of warships, troopships, supply vessels, and industrial assistance to the Cardassians from the Gamma Quadrant. Five such convoys entered the Alpha Quadrant completely unopposed by the Federation, Klingon, or Bajoran forces stationed on Deep Space 9, heavily fortifying the Cardassian Union. Tensions continued to escalate, resulting in Starfleet's decision to mine the entrance to the wormhole.